The invention relates to a method for automatically setting an operating point given signal curve measurements with a particle beam measuring apparatus in which primary particles impinge on a measuring location and an energy shift of secondary particles is triggered at the measuring location and is measured with an opposing field spectrometer. An opposing field of the opposing field spectrometer is set via a feedback circuit to a predetermined rated value for a specific reference potential at a measuring location.
Signal curve measurement with an electron beam measuring apparatus is based on the measurement of the energy shift of the secondary electrons with an opposing field spectrometer. Such a measurement of periodic signal curves with an opposing field spectrometer is disclosed, for example, in U.S. Pat. No. 4,220,853, incorporated herein by reference. Relative to a specific reference potential, the opposing field of the spectrometer is set to a predetermined value dependent on the respective distribution of the secondary electrons. Insofar as possible, this operating point should lie at the steepest location of that curve which describes the distribution of the secondary electrons. Such a setting of the operating point enables a high measuring precision. The distribution of the secondary electrons depends, among other things, on the material of which the measuring point is composed. The barrier of the opposing field spectrometer is usually set to a value between -6 V through -8 V with reference to ground potential. The setting of the barrier of the opposing field spectrometer occurs in a known feedback circuit (U.S. Pat. No. 4,277,679, incorporated herein by reference) by varying the gain of the secondary electron signal. Given the known method of the '679 patent, the measuring point for setting the barrier of the opposing field spectrometer is applied to ground potential and the gain of the secondary electron signal is then set via a variation of the voltage of a photomultiplier employed in the known feedback circuit. A method for setting the operating point according to U.S. Pat. No. 4,277,679 requires a changeover switch with which switching back and forth between a reference voltage and an operating voltage can be carried out, depending upon whether a setting of the operating point or a potential measurement is to be undertaken at the moment.
Given a method known from U.S. Pat. No. 4,220,854, incorporated herein by reference, for measuring the potential curve in an electronic component, the signal of a constant reference phase is measured together with and during the continuous sensing of a signal curve. The difference between the measured signal and the reference signal is thus free of drift, whereby both the measured signal as well as the reference signal respectively have drift phenomena in and of themselves, due to contamination at the measuring point.
A method for measuring signal curves is known from the German Patent Application No. P 32 35 484, incorporated herein by reference, wherein a known reference voltage is measured at a measuring location at at least one point in time for the suppression of disturbances in the signal curve, and wherein a secondary electron signal which results from a measurement of such a reference voltage then controls the operating point of a feedback circuit for the suppression of the disturbance.
All of the cited, known methods presume that the point in time at which a specific reference voltage is present at a measuring location is known. There is no indication in any of the cited publications as to how an operating point of a feedback circuit, given an electron beam measuring apparatus, can also be set when no single point in time at which a specific reference potential is present at a measuring point is known.